The present invention relates to a primary battery and more particularly, to a silver oxide primary cell of flat type including divalent silver oxide (AgO) as a main component of positive electrode (cathode) active material.
Following the recent progress in the field of electronics technology, various electronic instruments, such as electronic calculators, electronic wrist watches and the like have been generally reduced in size, and such a trend has now resulted in an increased demand for compact and small batteries, for example, button-type dry cells which may be employed as an external source of electric power to be installed in these electronic instruments.
Of the compact sized primary cells of the above described type, those having AgO as active material have been particularly required owing to their high energy density available despite of the small size thereof. However, divalent silver oxide primary cells fully satisfactory for the purpose have not conventionally been obtained as yet due to complication of structures, unstability of AgO, insufficient high rate discharge, etc. One of the problems inherent in the conventional silver oxide primary cells is that AgO tends to be decomposed in alkaline solution as in 2AgO.fwdarw.Ag.sub.2 O+1/2O.sub.2 to evolve gases, thus resulting in a possibility of destroying the cells. Countermeasures essential to solving such a problem as described above may reside in how to dispose AgO under circumstances where it is prevented from decomposition as well as obtaining stable AgO in the form of powder. However, most of the prior arts in connection with the above relate to suppression of decomposition by addition of various additives, suppression of decomposition through electrolyte concentration, restriction of area coming into contact with the electrolyte, etc. for stabilization of AgO itself, and refer to nothing about the countermeasures from the structural point of view. Meanwhile, another problem in the conventional silver oxide primary cells is related to the discharge capacity thereof, and in the primary cells employing AgO, it is not preferable to mix electrically conductive material as in the known cells using monovalent silver oxide (Ag.sub.2 O), with consequent large internal resistance.
Furthermore, the conventional silver oxide primary cells employing AgO have another disadvantage that in the AgO-Zn cells, they show two stage discharge reaction of approximately 1.8 V for AgO.fwdarw.Ag.sub.2 O and approximately 1.5 V for Ag.sub.2 O.fwdarw.Ag due to discharge potential of AgO.
In order to overcome the disadvantage as described above, there have conventionally been proposed various methods for obtaining a single potential discharge from AgO, in which the known facts that electrical contact between AgO and current collecting member through Ag.sub.2 O is required for achieving the single potential discharge and that such a state as described above spontaneously takes place in the process of AgO discharge reaction, are positively utilized for preliminarily providing structures suitable therefor.
The prior arts as described above include, for example, Japanese Laid Open Patent Application Tokkaisho 51-104534 in which, as shown in FIG. 6, the button type dry cell has a two-part container including a negative electrode (anode) container or cap 102, and a positive electrode (cathode) container or cup 101. An AgO layer 105 housed in the cup 101 is insulated from the cup 101 through a plastic layer 109, and simultaneously electrically contacts the cup 101 through a porous Ag layer Pb facing a negative electrode 103 which is accommodated in the cap 102, while the porous Ag layer Pb is electrically coupled with the AgO layer 105 through a porous layer Pa of monovalent silver oxide and also electrically connected to the cup 101 through a metallic annular contact member r, with a separator 107 and a liquid absorber 104 being further provided between the negative electrode 103 and AgO layer 105 and with an annular gasket 108 disposed between the cap 102 and cup 101 for sealing.
In the prior art disclosed in U.S. Pat. No. 3,655,450, the surface of an AgO pellet is covered with a continuous Ag.sub.2 O layer which contacts a positive electrode container, while AgO is physically separated from the positive electrode container. Meanwhile, in Japanese Laid Open Patent Publications Tokkaisho Nos. 51-18823 and 51-18824, there is provided a discontinuous oxidizable metallic screen or ring between a positive electrode active material and a vertical wall of the positive electrode container or between the positive electrode and a separator.
In the prior art of U.S. Pat. No. 3,655,450 referred to above, it is necessary to cover the entire surface of the AgO pellet with Ag.sub.2 O, for which there are proposed several methods. In one of these methods in which, for example, the pellet surrounded by a thin Ag.sub.2 O layer is forced into a cell container, there is a problem that the Ag.sub.2 O layer is broken through deformation of the pellet during the insertion thereof under pressure. Another method in which the pellet is formed into Ag.sub.2 O after pressing thereof into the container is advantageous in that the Ag.sub.2 O layer is not damaged, but employment of reducing agent for reduction of AgO into Ag.sub.2 O is required. Although U.S. Pat. No. 3,655,450 raises Zn, Cu, Ni and Ag as materials for the Ag.sub.2 O layer, reaction tends to be slow and uneven, if sufficient electrolyte is not present thereat. Furthermore, AgO in its property becomes very unstable upon contact with reduction agent such as metals, etc. so as to be readily decomposed, while most of these metals after oxidation in the form of oxides adversely affect the stability of AgO. Moreover, with the increase of area over which Ag.sub.2 O must be formed, the rate for utilization of the active material per volume is decreased, resulting in disadvantages in compact size batteries.
As described above, when AgO batteries are to be manufactured by the method of U.S. Pat. No. 3,655,450, there are involved many serious difficulties, with consequent necessity for high standard of quality control, and thus, the prior art method is not suitable for practical application.
Meanwhile, in Japanese Laid Open Patent Applications Tokkaisho Nos. 51-18823 and 51-18824 referred to earlier, the portion to be formed into Ag.sub.2 O is limited more than that in U.S. Pat. No. 3,655,450 so as to form the Ag.sub.2 O layer after insertion of the pellet into the container under pressure, and for the purpose, it is proposed to insert metallic materials such as Zn, Cu, Ag, Sn, Cd and Pb which can be readily oxidized. The known method as described above intends to fundamentally alter AgO into Ag.sub.2 O through electrochemical reduction and more specifically, to reduce AgO by the electric current produced through oxidizing reaction of oxidizable metals commonly contacting AgO and current collecting member owing to the so-called local cell reaction. Therefore, there is a possibility that, even when AgO is not continuously covered over its entire area, oxidation may extend over the portions which are not covered. In the case of the known method as described above, although reaction takes place rapidly as described in the specification thereof, the reaction is complicated depending on the relative positions of the oxidizable metals and current collecting member (i.e. positive electrode container), their quantities, shapes, state of penetration of electrolyte, etc., and all the portions of AgO contacting the positive electrode container are not necessarily formed into Ag.sub.2 O. The possibility of such inconveniences as described above may be anticipated from the statement in Japanese Laid Open Patent Application Tokkaisho No. 51-18823 that the initial discharging voltage by AgO is reduced to the discharging voltage by Ag.sub.2 O in a shorter period of time than in the absence of the oxidizable metals. Moreover, there are such disadvantages that the stability is remarkably deteriorated by the contact between the reducing metals and AgO and by oxides which are by-product of such contact, and also that the by-product material increases the internal resistance.
On the other hand, the arrangement of Japanese Laid Open Patent Application Tokkaisho No. 51-104534 mentioned earlier may be said to be more advantageous in the voltage stability than those in the foregoing prior arts in that, forming the large surface area of AgO into Ag.sub.2 O is not required, since the inner surface of the positive electrode container or cup is covered with the plastic layer. In the specification of the above known arrangement, a porous silver layer is disposed over the surface of the positive electrode, with a contact ring being employed for electrical conduction between the silver layer and positive electrode container. For producing the silver layer, there are disclosed two methods, and one of which is an electrochemical method through preliminary discharge only for the positive electrode, and the other of which is a chemical method through immersion of the positive electrode into a reducing solution. Both of the above described methods utilize the Ag layer obtained by the reduction of AgO, but the former method is disadvantageous in that a particular equipment is required for the purpose with unfavorable productivity, although the reaction amount is readily controlled, while in the latter method, the reaction amount is difficult to be regulated with uniform conductivity not being maintained in some cases, depending on the state of the layer formed, although mass-productivity thereof is generally favorable. Furthermore, disadvantages common to the both methods are such that, since immersion into the solution is effected before assembly of the battery, sufficient rinsing is required thereafter, for which rinsing process, much time is necessary, and if insufficient, solution leakage may take place after the assembly of the battery. Additionally, impurities in the rinsing water tend to affect adversely the stability of AgO. Furthermore, Japanese Laid Open Patent Application Tokkaisho No. 51-104534 as described above is very complicated in the arrangement thereof, and discloses the method for insertion under pressure either by partially covering only the lower half of the inner surface of the positive electrode container by the plastic film or by bending peripheral portions of sheet material piled up on the AgO pellet. In addition, it is essential that the contact ring uniformly contacts both the positive electrode container and AgO, and the technique required for the partial covering of the plastic film or physical state at the folded portion of the folded sheet tend to give rise to difficulties in manufacturing and unstability of quality of the resultant batteries, thus it being extremely difficult to produce particularly compact sized thin and flat batteries.